Birefringence—a difference in the refractive index exhibited by a material along two axes with respect to incident electromagnetic waves with different polarizations—can occur only in materials having an anisotropic (directionally dependent) structure. Birefringence is often used in single-mode optical fibers that transmit light signals. For example, so called “polarization-maintaining” (PM) fibers use birefringence to maintain the polarization state of incident light as it travels through the fiber.
The most effective way of introducing high birefringence while maintaining a circular, single-mode output is by imparting asymmetric stress in the cladding region of the fiber (i.e., the region that surrounds the light-conducting fiber core). Stress results from a difference in thermal-expansion coefficient along the two orthogonal axes of the fiber and is transmitted to the fiber core. As explained, for example, in U.S. Pat. No. 5,056,888 to Messerly et al., the disclosure of which is incorporated herein by reference in its entirety, an asymmetric stress-applying region can be combined with selective doping through the radius of the fiber to create a “W index profile” along one or both orthogonal axes. The depressed-index regions of the fiber (i.e., the lower points of the W profile where the refractive index is below that of the cladding) provide a tunneling loss that extinguishes the unguided polarization state, and which increases rapidly with wavelength. This may be achieved by doping so as to produce different coefficients of thermal expansion (CTEs) along two axes of the fiber; this, in turn, results in anisotropic stress that splits the mode-effective indices so that the cutoff wavelength differs for the two polarizations.
Polarization control has been an important design challenge in fiber-optic devices and systems. Most advanced sensing systems, communication systems, as well as fiber laser systems require control of the polarization state in the optical fiber. Traditionally, PM fibers have been widely utilized to maintain the polarization state of a pre-polarized signal. However, a PM fiber typically suffers from mechanical, geometrical and temperature perturbations along its length, resulting in a decrease of the polarization extinction ratio for polarized light over the length of the fiber (and hence a degradation of polarization control). Thus, even PM fibers with very high birefringence still guide two polarization states, and their ability to preserve the light polarization degrades over the length of the fiber due to cross-talk between two polarization modes; this can be caused, for example, by sensitivity to alignment to the source, as well as extrinsic perturbations experienced by the fiber itself.
PZ fibers, by contrast, propagate only one polarization state of a fundamental mode over a wide polarization bandwidth. An exemplary prior-art PZ fiber is described in the '888 patent, with refractive-index profiles for the two axes of this fiber appearing in FIG. 1. As shown, the refractive-index profiles for both the guided axis (x1) and the unguided axis (x2) in the PZ fiber are W-type profiles (i.e., the profiles exhibit a substantially “W” shaped profile with respect to the radius from the center of the fiber). More particularly, both profiles have a central region within the core of the fiber (i.e., for a radius up to ra) where the refractive index is greater than that of the refractive index of the cladding (ncl), an intermediate region outside the core region (i.e., from ra to rb, where rb corresponds to the outer radius of the intermediate doped region between the core and the cladding) where the refractive index has a depressed region lower than that of the cladding, and a cladding region extending out from the intermediate region. The cladding is formed, for example, from pure silica (having a refractive index, ncl, of approximately 1.457) or lightly doped silica (having a refractive index, ncl, of approximately 1.459). The refractive index along the guided axis generally differs from that of the unguided axis by a value δ1 (in the core region) and a value δ2 (in the intermediate region), with the unguided axis suffering more from leaky loss (which results in greater loss at shorter wavelengths), and the guided axis exhibiting significant leaky loss at longer wavelengths. As both axes have similar W-type profiles, these fibers merely maintain and guide the principle polarization modes along both axes, and cannot attenuate the light travelling therethrough to preserve only a single mode. In addition, under “macrobending” conditions (i.e., large bends that allow loss of light), both axes have significant leaky mode loss due to shifting of the refractive index, with the polarization bandwidth being significantly reduced under bending conditions.
A need exists, therefore, for an improved optical fiber capable of effectively overcoming the limitations of conventional PM and PZ fibers.